The present invention relates generally to ceramic heaters and more particularly, to a ceramic heater having a platinum heating element which is resistent to oxidation.
Ceramic heaters are generally known in the art. Normally, a ceramic heater will include an insulating portion, a heat generating portion, and electrical lead portions formed integrally with a ceramic body or substrate. The heater element and lead portions are normally formed of a single electrically conductive metal such as an inexpensive non-noble or base metal such as tungsten and molybdenum. However, the heater element and lead portions made of such metals are prone to oxidize during long periods of use at high operating temperatures in oxidizing atmospheres such as air. The oxidation may result in disconnection of the heat generating portion of the ceramic heater and, thus, heater failure.
The art has sought to solve this problem by decreasing the amount of non-noble or base metal used in the ceramic heater. For example, U.S. Pat. No. 4,952,903 to Shibata et al. (hereinafter xe2x80x9cShibata""3 teaches a ceramic heater including a ceramic body and a heater element formed of a cermet containing a ceramic material and a metal material which principally consists of at least one noble metal; and, including electrical lead portions formed of a metallic material consisting of at least one base metal or formed of a cermet containing ceramic material and metallic material. Shibata mentions the making of the heater element from a noble metal such as platinum or rhodium, but dismisses such use because of costs and the difficulty of bonding a noble metal to a ceramic substrate. The use of such noble metal would overcome the problems associated with oxidation of the metal. Thus, an economic and practical means of using such noble metals would be advantageous to the art of ceramic heaters. For these reasons, there remains room for improvement in the art.
It is an object of this invention to provide adequate binding of noble metals to a ceramic substrate.
It is also an object of this invention to provide a ceramic heater which does not require an outer sheath or cover and which is economical to manufacture.
It is another object of the present invention to provide a method of making a ceramic heater which provides for the screen printing of the heater element onto a ceramic sheet.
It is a further object of the present invention to provide a method of making a ceramic heater which does not require a cover layer to protect the heating element.
These and other objects of the invention are achieved by a ceramic heater comprising an alumina rod, an alumina based ribbon sintered to the rod, and a platinum resistor element bonded to the ribbon. These and other objects are also achieved by a method of making a ceramic heater comprising the steps of making a ceramic slurry; combining the ceramic slurry with a binder component to form a slip; depositing the slip onto a carrier film at a controlled thickness such that a deposited slip is formed; heat curing the deposited slip to form a cured slip ribbon; applying a platinum paste onto the ribbon in a specific pattern, the paste forming a platinum resistor element on the ribbon; applying the ribbon with the platinum resistor element onto an alumina rod; and, heating the rod with the ribbon and the platinum resistor element thereon, whereby the ribbon is sintered to the rod and the platinum resistor element is sintered and bonded to the ribbon.